1. Field of the Invention
This invention relates to improvements in heat-treating pulverous or granular raw material such as cement raw meal, lime and aluminum oxide, in a kiln plant.
2. Description of the Prior Art
In modern kiln plants, the kiln is usually of a rotary type and the process taking place in the kiln is generally sought to be limited to sintering of the materials to be treated, prior to feeding them to a cooler for cooling before further treatment. It is thus a prerequisite among other things to achieve maximum preheat-treatment in the preheating and/or calcining stage, followed by maximum cooling in the cooling stage. However, to achieve these ends, it has been found that heat exchange units of rather larger dimensions are required.
For example, the use of the heat in the spent cooling air of the preheat-treatment stage involves transporting the hot spent cooling air from one end of the kiln to the other, with the consequent inevitable heat loss, and the need for a large and efficient cooler and a large preheater and associated exhaust fans. Furthermore, if the material is subjected to a precalcination -- an endothermic process of expelling gases such as carbon dioxide -- in a calciner coupled to the preheater, the substantial amount of smoke produced by the burning of fuel in the calciner requires the use of large precipitators and even larger exhaust fans.
Japanese Patent Publication No. 3916 of 1963 relates to a process and kiln apparatus for burning cement wherein the heat transfer to the material delivered to the kiln is sought to be maximized. According to this process, cement raw materials are introduced into a rotary kiln either at the forward or at an intermediate position of the kiln and allowed to be heated and fluidized while flowing with the kiln gases toward the rear of the kiln. Thereafter the material is permitted to precipitate onto the kiln floor for further heat treatment in the kiln while moving along the inclined kiln bottom toward its foward end. British Pat. No. 1,396,402 relates to a process and device for treating particulate material by means of gases in a reaction chamber having a separator connected downstream of the gas flow. The material is passed into the reaction chamber as a suspension in a gas so as to be passed twice through the reaction chamber prior to being removed therefrom as a stream of material separated from the gas.
While these attempts improve somewhat the efficiency of the individual heating chambers, they nevertheless are not as effective as is desirable in improving the efficiency of the entire kiln plant while improving substantially its capacity. I have invented a method of heating such raw materials and a plant for practicing the method, in which the kiln plant capacity is substantially improved while the dimensions of at least certain portions of the preheater, calciner, cooler, smoke gas fans and precipitators are substantially decreased without increasing the power consumption of the plant.